A drill bit commonly used in the exploration and recovery of oil, gas and the like to bore through earth formation is the tri-cone rotary drill bit. In use, the tri-cone rotary drill bit is secured to the end of a string of drill pipe which rotates and urges the drill bit against the earth formation. The drill bit includes a drill bit body and a number of substantially conical shaped rock bit cutters rotatably supported by bearings at the pin end of the bit body.
As the drill bit is rotated, the bit cutters bore into the underlying earth formation. Extending radially from each bit cutter are a number of teeth, each having a tip made of diamond or metal that is harder than the underlying earth formation. The bit cutters essentially roll across the underlying earth formation as the bit rotates, causing the teeth extending from the cutter to create a bore hole by impacting and pulverizing the formation. At the same time drilling mud or air is pumped through the drill string and the bit body, around the bit cutters, to cool the cutters, lubricate exterior surfaces of the bit and sweep fragments of the pulverized formation from the cutters back to the surface through the space surrounding the drill string and drill bit.
The bearings supporting the rock bit cutter are protected from the highly abrasive mixture of drilling mud or air and cuttings, which would otherwise quickly destroy the bearings, by a bearing assembly that includes a bearing seal. Typically, bearing seals comprise an O-ring or a Belleville spring covered within an elastomeric material. The seal is positioned about the bearing surface and compressed between the bearing surface of the bit body and the bearing surface of the bit cutter to form a barrier against intrusion of drilling mud, air and cuttings.
Early Belleville spring seals for tri-cone rock bits were designed with a metallic Belleville spring clad with an elastomer, usually nitrile rubber. The metallic Belleville spring provided the energizing force for engaging the sealing surface, and the rubber coating sealed against the bearing surface of the bit body and the bearing surface of the bit cutters. This provided a seal on relatively rough surfaces because the compliant behavior of the rubber coating filled in the microscopic asperities on the sealing surface. The seal would fail after a relatively short number of hours in operation due to wear of the elastomer, resulting in loss of lubricant contained within the bearing cavity. The bit would continue to function for some period of time utilizing bearings without benefit of the lubricant.
While O-ring type seal assemblies have improved the operation of the typical rotating cutter drill bit, the seal assembly has an effective service life typically within the range of 100-200 hours of drilling. The seal assembly will degrade and permit drilling mud and other fluids present about the drill bit to enter the bearing assembly. Once the bearing assembly is contaminated, the service life of the drill bit is very short. The drilling mud and other fluids about the drill bit commonly contain impurities which quickly abrade the bearing surfaces of the bearing assembly.
However, a significant advancement in rock bit seals came when O-ring type seals were introduced. These seals were composed of nitrile rubber and generally had a circular cross-section. The seal was fitted into the radial gland formed by the cylindrical surfaces between the pin end and the cutter bearing. The annulus formed was smaller than the original measured dimension of the cross-section of the O-ring seal. The squeeze of the seal was defined as the percentage reduction of the cross-section from its original shape to the deflected shape.
A variation of the conventional O-ring seal was an elongation of the radial dimension which, when compared with the O-ring seal, required less percentage squeeze to form an effective seal. Several other minor variations of the O-ring seal have been used, each relying on an elastomer seal squeezed radially in a gland formed by surfaces between the two bearing elements.
There are drawbacks to the conventional O-ring seal. As mentioned, the dynamic sealing component is composed of either nitrile rubber or hydrogenated nitrile rubber compounds. This material imparts a very high coefficient of friction when moving against steel or other metallic surfaces. This high friction characteristic causes the seal to generate heat, thereby raising the temperature of the rubber and reduces the physical properties of the materials, causing accelerated wear of the seal. An additional shortcoming of the conventional O-ring seal design is that the positive nature of the seal acts to prevent lubricant from getting to the seal interface during operation. The seal is lubricated during installation, and when this initial lubricant is expended, little or no additional lubrication of the seal interface is possible. Dry friction between the seal interface with the bearing gland acts to accelerate the wear of both the seal and the gland material, causing loss of the required squeeze to maintain effective sealing. To overcome this dry interface problem internal lubricants, such as graphite, have been blended into the seal compound to provide lubrication of the seal interface after the installation lubricant is expended.